In combustion of coal in furnaces such as in utility boilers, a large amount of ash is produced from the mineral matters originally contained in coal. The ash content of coal ranges from a few percent to over 30%, but the bulk of bituminous coal used for power generation in the U.S. has ash content within the range of 6 to 20. Ash generated in utility boilers is often sold as raw material for cement manufacture and for other processes. If the residual carbon content in ash is more than several percent, it is not acceptable for cement manufacture due to its color and its impact on the properties of cement. Ash with high carbon content, if no other uses could be found, is land filled at a significant cost.
Another concern with high carbon content in ash is potential increase in slagging and fouling tendency by ash. It has previously been stated that if carbonaceous residues become embedded in the deposits, a locally reducing atmosphere can be created in the deposit which can lead to a significant depression of slag melting point temperatures, especially if the ash is of high iron content.
The residual carbon content in ash is strongly influenced by the combustion conditions as well as the type of coal and mineral matters and the fineness of pulverized coal particles. In general the carbon content in ash increases with decreasing excess air for combustion. On the other hand, NOx emissions decrease significantly with decreasing excess air. Due to the more stringent emissions regulations for NOx, many coal fired combustion systems have been converted to low NOx combustion systems, in which combustion takes place in two stages, a fuel rich first stage where fuel bound nitrogen species are converted to molecular nitrogen and a fuel lean second stage where additional combustion air is mixed to complete combustion. The fuel rich combustion process used in low NOx combustion methods delay the combustion of char and tends to increase carbon content of ash at the furnace exit. Thus low NOx combustion methods tend to cause problems in the quality of ash as a salable by-product.
In combustion of coal, volatile combustion is very rapid and char combustion is slow. The unburned carbon content (UBC) in ash is determined by the slower char combustion step. The rate of carbon burnout depends on the particle size, pore structure, reactivity of char, temperature and partial pressure of oxygen in the atmosphere, among other things. Rapid mixing of coal and combustion air and more intense combustion with high excess oxygen are effective in reducing UBC. However, these conditions tend to increase NOx emissions sharply. Thus, design compromise is generally made in the burner-boiler design to achieve acceptable UBC while minimizing NOx emissions.
Ultra fine grinding of coal is an effective way of accelerating the char burn out and to reduce UBC. However, the cost of new coal pulverizing machine and the extra energy required for ultra fine grinding often make it uneconomic for most retrofit applications. Post combustion treatment of ash is possible and technologies exist to oxidize the residual carbon in ash in a separate process. However, post combustion treatment is expensive and not widely practiced.
Oxygen enrichment was tried in a test furnace fired with a coal-water slurry for NOx and carbon emission control under unstaged conditions. (W. F. Farmayan, et.al., “NOx and Carbon Emission Control in Coal-Water Slurry Combustion”, Sixth International Symposium on Coal Slurry Combustion and Technology, Orlando, Fla., Jun. 25–27, 1984.) Oxygen was injected from four injection tubes (about ¼″ ID) around the center fuel pipe. (FIG. 4 of the paper) The amount of oxygen injection was equivalent to oxygen enrichment of combustion air to 22 to 24% O2. This paper concluded that the effect of O2 enrichment near the burner on carbon burnout was found to be weak, particularly toward the flame tail end. The reason is thought to be that the relatively large unburned particles experience the same oxygen-depleted environment at the flame tail end as would those in a flame without oxygen enrichment. It is expected that more benefit would be gained from the point of view of carbon burnout by injection of O2 where it is needed most, e.g., behind the flame front rather than before.
Comparison of the oxygen injection method and results from this reference to the present invention has to consider the difference in burner configurations and fuel property. Coal water slurries (CWS) typically contain about 30% water and have to be atomized for combustion. The paper concluded that atomization quality of the slurry was the critical variable affecting carbon burn out. Unlike the traditional pulverized coal combustion, CWS must be atomized, and the droplets containing multiple coal particles must vaporize water before coal devolatilization and ignition can take place. Multiple coal particles in a single water droplet may agglomerate to form a larger particle. Thus the results from this reference cannot be applicable directly to pulverized coal combustion.
U.S. Pat. No. 4,495,874 discloses oxygen enrichment of primary and/or secondary air in pulverized coal fired burners in order to increase the steam rate of a boiler firing high ash pulverized coal. Although the patent does not teach the effects of oxygen enrichment on UBC, FIG. 4 shows significant increases in furnace temperatures near the burner with 2% oxygen enrichment, but little with 1% oxygen enrichment.
U.S. Pat. No. 4,596,198 discloses oxygen enrichment of primary air in pulverized coal fired burners in order to reduce slag deposition in coal fired utility boilers. The patent teaches 1% to 7% oxygen enrichment, preferably 2 to 5% enrichment, (which as defined in that patent is based on the oxygen addition to the total amount of combustion air) of primary air to reduce slag deposition. Furthermore it states that oxygen enrichment of 1% offers little, if any, benefit in slagging reduction.
O. Marin, et.al., discuss the benefits of oxygen for coal combustion in a paper entitled “Oxygen Enrichment in Boiler” (2001 AFRC/JFRC/IEA Joint International Combustion Symposium, Kaui, Hi., Sep. 9–13, 2001). They proposed injection of oxygen in the over fire air (also described as “tertiary air” in this paper), to reduce unburned carbon in ash, or Loss on Ignition (LOI), without increasing NOx emission. The computer simulation results reported by Marin, et al. compared the baseline air case and an oxygen enriched case with a high velocity, oxygen enriched stream in the tertiary air (also termed over-fire air). According to Marin, et.al., “An increase of 5% on heat transfer in the combustion chamber, combined with a 7% absolute increase in char burnout are noted.” (page 8)